Multiple-input multiple-output (MIMO) communication techniques exploit performance gains achieved by using multiple transmit and receive antennas within a system to provide un-correlated propagation channels between a transmitter and receiver. Typically, it is the correlation between different antenna elements (e.g., propagation paths) that enables multiple-input multiple-output techniques to realize advantageous performance in a realistic usage environment. Such performance advantages include increased throughput and operating range at the same bandwidth and same overall transmit power as other prior communications techniques.
During testing and development of multiple-input multiple-output communications equipment, channel emulators are sometimes employed to simulate usage conditions. It is desirable that a channel emulator be able to simulate realistic multiple-input multiple-output scenarios with accuracy, repeatability and performance that does not limit the performance (or apparent performance) of a device under test (DUT). At least one known multiple-input multiple-output channel emulator is based on: down-conversion of a signal from radio frequency to baseband; conversion of baseband signal from analog to digital; application of a predetermined baseband channel model (i.e., simulation scenario); conversion of the model signal from digital back to analog; and up-conversion of the analog signal from baseband back to radio frequency.
Under the known operational sequence outlined above, the channel model is applied digitally at a baseband sample rate, thus permitting such a channel emulator to apply virtually any sophisticated, dynamically varying channel model. However, this known approach also introduces noise and distortion at each step in the sequence described above, resulting in a noise floor on the signal that may limit the ultimate performance of the device under test and/or provide misleading indications as to one or more aspects the devices overall behavior. In some situations—including almost all high data rate scenarios—this noise floor problem is such that a device or system under scrutiny cannot be fully validated. These known channel emulators also tend to be relatively expensive, with some units exceeding $500,000 in cost, while being limited to a four-transmit/four-receive channel operation.